Chemistry Lab C

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Re: Chemistry Lab C

Post by jaah5211 » February 18th, 2019, 6:17 pm

Oopss totally forgot to ask the next question...

Which proton in ascorbic acid is the most acidic?


Why is that so?
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Re: Chemistry Lab C

Post by Cathy-TJ » February 21st, 2019, 7:31 pm

jaah5211 wrote:Oopss totally forgot to ask the next question...

Which proton in ascorbic acid is the most acidic?


Why is that so?
The most acidic proton is going to be the one that makes the most stable conjugate base. In this case, removal of the proton on the 4th carbon of the furan ring results in three possible resonance structures, while removing the others results in two. 
[img]https://i.stack.imgur.com/KVCSd.png[/img]
This is a good illustration of the three resonance structures.
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Re: Chemistry Lab C

Post by whythelongface » February 21st, 2019, 8:20 pm

Cathy-TJ wrote:
The most acidic proton is going to be the one that makes the most stable conjugate base. In this case, removal of the proton on the 4th carbon of the furan ring results in three possible resonance structures, while removing the others results in two. 
[img]https://i.stack.imgur.com/KVCSd.png[/img]
This is a good illustration of the three resonance structures.
I don't like the second of the three resonance structures. Even stabilized, carbanions are inherently unstable (a lot more so than oxyanions) and I don't think it contributes to the stability of the ascorbate ion in any major way.
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Re: Chemistry Lab C

Post by Cathy-TJ » February 21st, 2019, 9:00 pm

whythelongface wrote:
Cathy-TJ wrote:
The most acidic proton is going to be the one that makes the most stable conjugate base. In this case, removal of the proton on the 4th carbon of the furan ring results in three possible resonance structures, while removing the others results in two. 
[img]https://i.stack.imgur.com/KVCSd.png[/img]
This is a good illustration of the three resonance structures.
I don't like the second of the three resonance structures. Even stabilized, carbanions are inherently unstable (a lot more so than oxyanions) and I don't think it contributes to the stability of the ascorbate ion in any major way.

That's a good point.
Looking over the possible structures again, I still think that proton is the most acidic, since it has two stable resonance structures, while the other proton on the OH of the third carbon in the ring only has one. Correct me if I'm wrong.
Thanks for catching that!
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Last edited by Cathy-TJ on February 21st, 2019, 9:08 pm, edited 3 times in total.
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Re: Chemistry Lab C

Post by whythelongface » February 21st, 2019, 9:02 pm

Cathy-TJ wrote: That's a good point.
Looking over the possible structures again, I still think that proton is the most acidic, since it has two stable resonance structures, while the other proton on the OH of the third carbon in the ring only has one. Correct me if I'm wrong.
Thanks for catching that!
Pretty sure the conclusion is correct! Google confirms this. Anyway, you should ask the next question!
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Re: Chemistry Lab C

Post by Cathy-TJ » February 22nd, 2019, 2:57 pm

a) What two substances mixed together make aqua regia? In what relative concentrations?
b) Describe how aqua regia dissolves gold, and write a balanced equation for the reaction.
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Re: Chemistry Lab C

Post by wec01 » February 26th, 2019, 2:58 pm

Cathy-TJ wrote:a) What two substances mixed together make aqua regia? In what relative concentrations?
b) Describe how aqua regia dissolves gold, and write a balanced equation for the reaction.
a) 3 parts hydrochloric acid, 1 part nitric acid
b) The nitric acid oxidizes the gold while the hydrochloric acid provides chloride ions.
[math]Au + HNO_3 + 4 HCl \rightarrow [AuCl_4]^- + [NO] + [H_3O]^+ + H_2O[/math]
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Re: Chemistry Lab C

Post by Cathy-TJ » February 26th, 2019, 4:38 pm

wec01 wrote:
Cathy-TJ wrote:a) What two substances mixed together make aqua regia? In what relative concentrations?
b) Describe how aqua regia dissolves gold, and write a balanced equation for the reaction.
a) 3 parts hydrochloric acid, 1 part nitric acid
b) The nitric acid oxidizes the gold while the hydrochloric acid provides chloride ions.
[math]Au + HNO_3 + 4 HCl \rightarrow [AuCl_4]^- + [NO] + [H_3O]^+ + H_2O[/math]
Looks good! You have the next question.
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Re: Chemistry Lab C

Post by wec01 » February 26th, 2019, 5:59 pm

1. Write a balanced chemical equation for the reaction between calcium carbonate and hydrochloric acid.
2. Is this reaction exothermic or endothermic?
3. What molecule is produced that most greatly contributes to the answer in part 2?
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Re: Chemistry Lab C

Post by BrownieInMotion » February 27th, 2019, 4:18 pm

wec01 wrote:1. Write a balanced chemical equation for the reaction between calcium carbonate and hydrochloric acid.
2. Is this reaction exothermic or endothermic?
3. What molecule is produced that most greatly contributes to the answer in part 2?
1. [math]CaCO_3 + 2HCl \rightarrow H_2O + CO_2 + CaCl_2[/math]
2. Exothermic.
3. CaCl2? Forming ionic bonds is usually very exothermic so CaCl2 should have a very negative Hf.
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